The use of stretch-forming techniques to change the shape of metals is well known. In stretch-forming, the work piece is elongated under tension so that the elastic limit is exceeded, resulting in a permanent deformation of the work piece during the forming process. The summation of stresses due to bending and stretching are sufficient to cause yielding to take place in the material. At the same time, the strain values must not exceed limitations imposed by the ductility of the material. For many materials, the margin between the minimum strain required to effect permanent deformation and the maximum strain permissible to prevent breakage or tearing of the material may be quite small.
A typical stretch-forming machine includes a form die around which the work piece is wrapped under tension to form the work piece into the desired shape. Wrapping is preferably accomplished by a pair of wrap arms, each of which are independently driven through a predetermined path on either side of the die. Hydraulic tension cylinders are pivotally supported on the ends of the arms and are provided with clamping means for gripping the ends of the work piece. The hydraulic cylinders control the tension applied to the work piece while the movement of the control arms causes the work piece to be wrapped around the forming surface of the die.
Various control systems have been devised for maintaining sufficient tension on the work piece to stress the material beyond the yield point without causing material failure. One such system, described in U.S. Pat. No. 2,824,594, is known as the "positive position" control system. This system uses servo control mechanisms to control the positions of the gripping jaws during the wrapping operation. A wire is attached to and wraps around a contoured template mounted above the die as the arms are moved to wrap the work piece around the die. The wire terminates on reels mounted on the gripping jaws. Sensors react to any change in length of the wire to operate the servo units. The servo units control the hydraulic cylinders to maintain the length of the wire between the position sensors mounted on the gripping jaws at a constant length during the wrapping operation. Forming parameters can be modified by varying the contour of the template.
The positive position system has the disadvantage that a separate template must be used for the wire. Moreover the wire must be present during the active forming process. The installation of the template on the press is time consuming, and the template as well as the cable can impede part manipulation and accessibility.
An alternative control system heretofore proposed provides open-ended control of the position of the gripping jaws as a function of control arm position during the wrapping operation. Such a control system includes means for recording the movement of both tension cylinders as a function of angular position of the control arms. Once one piece is formed and the data required is recorded, the recorded data can be played back to control the stretch-forming machine to reproduce additional parts. Such an arrangement has the disadvantage of requiring an actual work piece to be present during the Record mode.